fix: Replace theme dropdown with intuitive toggle button

[shadow-dawg]

Problem Statement

The Ergo application currently uses a theme dropdown menu in the navigation bar, which creates unnecessary friction in the user experience. Users must:

Locate and click the theme selector icon
Wait for dropdown menu to open
Select their preferred theme from the list
Click to confirm selection

This multi-step process is inefficient for a feature that users interact with frequently.

I am solving this issue